How city living is reshaping the brains and behaviour of urban animals

When next you meet a rat or raccoon on the streets of your city,
or see a starling or sparrow on a suburban lawn, take a moment to
ask: where did they come from, so to speak? And where are they
going?

In evolutionary terms, the urban environments we take for
granted represent radical ecological upheavals, the sort of massive
changes that for most of Earth's history have played out over
geological time, not a few hundred years.

Houses, roads, landscaping, and the vast, dense populations of
hairless bipedal apes responsible for it: all this is new, and
animals are adapting, fast, all around us. A growing body of
scientific evidence suggests that the brains and behaviours of
urban animals are changing rapidly in response.

"Humans are creating all these totally new environments
compared to what they've seen in evolutionary history"

Emilie
Snell-Rood, University of Minnesota

"A lot of biologists are really interested in how animals are
going to deal with changes in their environments," said biologist
Emilie Snell-Rood of the University of Minnesota. "Humans are
creating all these totally new environments compared to what
they've seen in evolutionary history."

Snell-Rood is one of many researchers who have updated the
conventional narrative of urban animals, in which city life favours
a few tough, adaptable jack-of-all-trades -- hello, crows! -- and
those species fortunate enough to have found a built environment
similar to their native niches, such as the formerly
cliff-dwelling rock doves we now call pigeons and find perched on
building ledges everywhere.

The long view, though, is rather more multidimensional. Cities
are just one more setting for evolution, a new set of selection
pressures. Those adaptable early immigrants, and other species that
once avoided cities but are slowly moving in, are changing
fast.

As Snell-Rood and colleagues describe in a 21
August Proceedings of the Royal Society
B article, museum specimens gathered across the 20th
century show that Minnesota's urbanised small mammals -- shrews and
voles, bats and squirrels, mice and gophers -- experienced a jump in brain size compared to rural mammals.

Snell-Rood thinks this might reflect the cognitive demands of
adjusting to changing food sources, threats, and landscapes. "Being
highly cognitive might give some animals a push, so they can deal
with these new environments," she said.

Brain size is, to be sure, a very rough metric, one that's been
discredited as a measure of raw intelligence in humans. For it to
fluctuate across a whole suite of species, though, especially when
other parts of their anatomy didn't change, at least hints
that something cognitive was going on.

Many other studies have looked at behaviour rather than raw
cranial capacity. In these, a common theme of emerges: urban animals tend to be bold, not backing down from threats
that would send their country counterparts into retreat. Yet even
as they're bold in certain situations, urban animals are often
quite wary in others, especially when confronted with something
they haven't seen before.

"Most of the birds that never approach new objects or enter new
environments in this long period of time are urban," Miranda said.
"There are many new dangers in a town for a bird. Cars can run you
over. Cats can eat you. Kids can take you home."

Somewhat counterintuitively, bold urban animals also tend to be
less-than-typically aggressive, a pattern documented in species as
disparate as house sparrows and salamanders, the latter of which are a
specialty of Jason Munshi-South, an evolutionary biologist at the
City University of New York. The city's salamanders -- there aren't
many, but they're there -- "tend to be languid," said
Munshi-South. "If you try to pick them up, they don't try to escape
as vigorously as they do outside the city. I wonder if there's been
natural selection for that."

If so, it might be driven by high population densities of
salamanders in the city. Aggressive neighbours don't tend to be
good neighbours. Through that lens, city animals could be
domesticating themselves, a process that can occur without direct human intervention.

Even more fundamentally, muted stress responses have been found
in many species of urban animals. When surprised or threatened,
their endocrine systems release lower-than-usual amounts of stress
hormones. It's a sensible-seeming adaptation. A rat that gets
anxious every time a subway train rolls past won't be very
successful.

"They're clearly attenuating their physiological response to
stress, probably because they're constantly inundated with noise,
traffic, and all kinds of environmental stresses in cities," said
biologist Jonathan Atwell of Indiana University. "If they were
ramping that response up all the time, it would be too costly."

A challenging question is whether traits like these represent
inherited biological changes or what researchers call phenotypic
plasticity: the ability to make on-the-fly adjustments to
circumstance.

Some adaptations, such as the swath of genetic mutations that
Munshi-South identified in New York
City's white-footed mice, are clearly heritable. Others are
learned. In many cases, both processes are likely involved, said
Atwell, who studied the question in his research on songbirds
called dark-eyed juncos around San Diego.

The San Diego juncos sing at higher frequencies than those
living in rural, traffic-free settings. When Atwell raised some of
their chicks in a quiet place, that rise in song frequency dropped by about half, suggesting an
even split between heritability and plasticity.

Where things get really interesting, though, is with social
learning and animal
culture -- all those animal habits and abilities that are
not inborn, but taught. "I suspect that often it's not their
cognitive abilities evolving, but cultural evolution going on,"
said Atwell. "Anytime animals can learn behaviour from one another,
I think there might be cultural evolution."

"You could imagine some kind of speciation over long
periods of time"

Emilie Snell-Rood, University
of Minnesota

Urban squirrels, for example, seem to have adjusted to
vocalisation-drowning ambient noise by making
tail-waving a routine part of communications. Perhaps this was
instinctive in a few animals, then picked up by others. Likewise,
squirrels might learn about traffic by seeing others get run over,
said Snell-Rood. Rats could see brethren die after eating poisoned
bait, then teach pups to avoid the traps.

Not all changes in urban animals will represent adaptations to
urban living, however.

Most genetic mutations are neither beneficial nor harmful, at
least not right away. They simply happen and, over long
periods of time, accumulate in populations through what's known as
genetic drift. In isolated groups, drift's effects are magnified,
as are so-called founder effects, in which entire populations bear
the genetic imprint of a few early animals. For these creatures,
urban adaptations won't necessarily represent adjustments to city
life, but simple happenstance.

How might this play out in deep time? If humans can keep
civilisation intact long enough, will urban animal populations
eventually become their own distinct species -- bold, relaxed, and
clever, with a store of learned information about our habits, and
perhaps a few other traits that arise by chance?

Nobody knows, said Snell-Rood, but "you could imagine some kind
of speciation over long periods of time." She noted, though, that
not all the changes seen in urban animals are necessarily
permanent. The big brains of those city-dwelling Minnesota mammals,
for instance, seemed to shrink after a few decades of urban
adaptation.

"The way I interpret it is that during the initial colonisation,
it pays to be smart," Snell-Rood said. Once city life becomes
predictable, "you can go back to having a smaller brain."